This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2002-197697, filed on Jul. 5, 2002, and No. 2001-227314, filed on Jul. 27, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method and a photo mask for manufacturing an array substrate.
2. Related Background Art
Liquid crystal display devices (hereinbelow also called LCD) have recently brought into wide use in personal computers, projection-type television sets, compact television sets, portable information terminals, and so on. The main stream of currently existing LCDs is active matrix LCD in which thin-film transistors (hereinbelow also called TFT), which are semiconductor elements, are provided for individual pixels.
Active matrix LCD is made up by confining a liquid crystal between an array substrate having a display electrode and a filter substrate having a common electrode opposed to the display electrode. A TFT array substrate having TFTs in a matrix arrangement is frequently used as such array substrate. The TFT array substrate has a plurality of signal lines connected to TFT sources, and a plurality of scanning lines connected to TFT gates, which intersect in form of a grating. As the active layer of TFT, amorphous silicon or polysilicon is used.
If polysilicon having a larger mobility than amorphous silicon is employed as the semiconductor material, part of the drive circuit for displaying images can be formed on the array substrate. As a result, some parts having been attached externally of a cell panel can be omitted. This resulted in lowering the manufacturing cost and a compact outer frame of the LCD display.
If more drive circuits are built on the array substrate, its cost will be further lowered and the function will be enhanced.
However, array substrates using currently available polysilicon as their semiconductor material still allow only a limited number of drive circuits to be built on. Therefore, circuits other than those built on the substrate are still located externally of the array substrate.
To build more drive circuits on an array substrate, mobility of polysilicon is preferably high. Increasing the grain size of polysilicon would improve the mobility of the polysilicon.
There is a method for enlarging the grain size of polysilicon by irradiating energy beams such as laser beams onto an amorphous silicon film, there by producing solid/liquid interface, and using a temperature profile along the interface to grow the crystal laterally in parallel to the plane of the array substrate. This method is called the lateral growth method.
The lateral growth method irradiates energy beams such as laser beams on an initial film on the substrate via a photo mask, for example. In this case, crystal growth direction depends on the profile of the energy beams formed by the photo mask.
FIG. 7A is a fragmentary, enlarged view of a conventional photo mask 100. The photo mask 100 includes rectangular transparent regions 10 and shutoff regions 20. The energy beams passing through the aperture 10 melt the amorphous silicon (or polysilicon). Once the irradiation of energy beams is completed, crystal grows from the interface between solid phase portions and liquid phase portions of silicon (hereinbelow also called solid-liquid interface) toward the inside.
FIG. 7B is an enlarged plan view of crystal grains of polysilicon after irradiation of energy beams. In the lateral growth process, crystal grows from the solid-liquid interface. Thus the crystal growth direction is different between the short side and the long side of the transparent region 10. Therefore, crystal grains 30 grown from short side and crystal grains 40 grown from the long side are different in lengthwise direction of crystal grains. Especially because the transparent region 10 was rectangular, lengthwise directions of the crystal grains 30 and the crystal grains 40 were intersecting approximately at a right angle.
FIG. 8 is a plan view that schematically shows placement of TFTs 60, 70, 80, 90 formed by using conventional polysilicon as their active layers 50. TFTs 60, 70, 80, 90 each include a gate electrode 110, source electrode 120 and drain electrode 130.
When a voltage is applied to the gate electrode 110, each TFT turns ON. That is, the active layer under the gate electrode 110 reverses, and forms a channel. The channel allows a current to flow between the source electrode 120 and the drain electrode 130.
While TFTs 60, 70, 80, 90 are OFF, the current leaking out between each source electrode 120 and the associated drain electrode 130 had better be small. On the other hand, when the TFTs 60, 70, 80, 90 are ON, the resistance value (referred to as ON resistance) between each source electrode 120 and the associated drain electrode 130 had better below. Further, TFTs 60, 70, 80, 90 preferably have constant properties.
In general, when the flow direction of carriers of TFT substantially coincides with the lengthwise direction of polysilicon crystal grains, carriers exhibit a higher mobility. As the mobility of carriers is high, the ON resistance decreases. On the other hand, as the flow direction of carriers deviates from the lengthwise direction of crystal grains toward 90 degrees therefrom, the mobility of carriers becomes lower because carriers must pass through more grain boundaries and more of them will be scattered.
In the conventional polysilicon active layer 50, because the transparent region 10 is formed rectangle, lengthwise directions of crystal grains 30 and 40 intersect substantially at a right angle. Therefore, the conventional technique has the problem that carrier mobility is relatively low in TFT 90, although it is relatively high in the other TFTs 60, 70 and 80.
The conventional technique also has the problem that TFTs 60, 70, 80, and 90 cannot exhibit constant properties.
Attempts to prevent those problems invite a design constraint that disables TFTs to be formed in regions where crystal grains 30 exist. Further, for forming TFTs in regions where crystal grains 30 do not exist, the manufacturing process will need an additional process for positional alignment.
According to an embodiment of the invention, there is provided a method of manufacturing an array substrate comprising:
depositing an amorphous material on a transparent substrate; and
changing the amorphous material to a polycrystalline material by irradiation of energy beams through a photo mask, the mask including a transparent region permitting the energy beams to pass through and a shutoff region surrounding the transparent region and interrupting the energy beams, the transparent region being defined by first and second lengthwise direction lines extending substantially in parallel to each other, first and second slanting direction lines which extend from opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other; and third and fourth slanting direction lines which extend from the other opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other, the transparent region having a length in the extending direction of the first and second lengthwise direction lines, which is longer than the length of the transparent region in the direction perpendicular to the extending direction of the first and second lengthwise direction lines,
wherein changing the amorphous material to the polycrystalline material includes: moving the transparent substrate by a constant distance perpendicularly to the lengthwise direction of a flat pattern projected onto the surface of the amorphous material when energy beams passing through the transparent region are irradiated onto the amorphous material; and irradiating the energy beams onto the amorphous material every time when the transparent substrate is moved.
According to a further embodiment of the invention, there is provided a method of manufacturing an array substrate comprising:
depositing an amorphous material on a transparent substrate; and
changing the amorphous material to a polycrystalline material made of crystal grains by irradiation of energy beams through a photo mask permitting the energy beams to pass through, the photo mask including an elongated transparent region configured to permit the crystal grains to grow in directions not crossing at right angles when the energy beams are irradiated onto the amorphous material, the photo mask further including a shutoff region surrounding the transparent region to interrupt the energy beams, wherein changing the amorphous material to a polycrystalline material includes: moving the transparent substrate by a constant distance perpendicularly to the lengthwise direction of a flat pattern projected onto the surface of the amorphous material when energy beams pass through the transparent region and are irradiated onto the amorphous material; and irradiating the energy beams onto the amorphous material every time when the transparent substrate is moved.
According to a still further embodiment of the invention, there is provided a photo mask permitting energy beams emitted from an energy source to pass through to change an amorphous material to a polycrystalline material, comprising:
a transparent region permitting the energy beams to pass through and defined by first and second lengthwise direction lines extending substantially in parallel to each other, first and second slanting direction lines which extend from opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other, and third and fourth slanting direction lines which extend from the other opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other; and
a shutoff region surrounding the transparent region to interrupt the energy beams,
wherein the transparent region has a length in the extending direction of the first and second lengthwise direction lines, which is longer than the length of the transparent region in the direction perpendicular to the extending direction of the first and second lengthwise direction lines.